As it is known, the processes for manufacturing and inspecting workpieces involve the use of master parts adapted to provide datums or references for the critical features or surfaces of the workpieces, in particular for linear dimensions, shape and geometry features, etc.
In particular, these master parts are necessary for zero-setting devices and machines used for checking, through comparative measurements, said features of the workpieces during the different steps and at the end of the mechanical machining operations relating to the same workpieces.
The known master parts are manufactured through precision, chip-forming machining operations, on blocks of special steel for master parts, at least as far as the surfaces corresponding to the critical surfaces of the workpieces are concerned. After the mechanical machinings, the master part undergoes hardening and annealing heat treatments.
The material of the conventional master pats is characterized by very high hardness and stability and therefore this material is "special" for both intrinsic chemical features and the hardening and annealing treatments.
These master parts have conceptually monolithic, i.e. integral structure, even if sometimes--due to accessibility problems or other practical difficulties--, especially in the case of very complex master parts, this structure is comprised of a plurality of blocks.
In order to preserve the master parts from corrosion, often on them there are applied, before the finishing machinings and the final heat treatments, chromium or nickel platings, or platings with other anticorrosive materials. This renders the finishing (grinding, lapping, etc.) machinings more difficult and expensive, in particular with regard to the reference surfaces, where consequently these platings are not always applied. In this case, the reference surfaces must periodically be greased.
Moreover, the master parts are subjected to a certification procedure in a metrology department, by means of measuring apparatuses having a very high accuracy. This procedure is basically distinct from the manufacturing process.
The manufacturing of a conventional master part is very complex and expensive, since, among other things, a machining error may cause the rejection of the entire master part or of a substantial part of it. Besides, the manufacturing process has a very poor or null flexibility, because the major part of it is specific for a determined workpiece.
Usually, the shape of the master parts is rather similar to that of the relevant workpieces, especially as far as the critical surfaces of the workpieces are concerned.
As it is known, even if the workpieces have to be dynamically checked (for example, in the case of shafts, during rotation about their geometrical axis), the master parts are used in a static way, by using as references only a minor portion of the precision machined surfaces or zones. Thus, for example, when cylindrical surfaces providing references or datums for corresponding workpiece surfaces are concerned, two or four points or zones of these surfaces of the master part are used, to define one or, respectively, two diameters.